Examples of industrial applications for dissolution of a metal, such as Zn or Sn in an aqueous electrolyte are the preparing of a solution containing ions of zinc or tin for electroplating purposes or dezincing or detinning of metal scrap, especially of steel scrap.
For preparing or replenishing a solution for plating purposes a metal may be fed to the aqueous electrolyte in a substantially pure form and no selective dissolution with respect to other metals may be needed. For removing the metal from e.g. steel scrap selective dissolution is desired, selective in that only the metal(s) to be dissolved dissolve, in order to be able to separate the metal from the scrap thus obtaining the separate metal and steel scrap substantially free of that metal.
As Sn and Zn may be applied as a covering layer on steel by electroplating from an electrolyte and the resulting steel is to be recycled, both above-mentioned dissolution applications are particularly relevant for steel production and recycling. Tinned steel is a widely used packaging material and galvanized steel is used in numerous product applications for example in automotive applications. In an electrochemical process for selective dissolution of Zn and Sn these metals and also Pb and Al may be separated from steel scrap, thus providing for recovery of such metals as well as clean steel scrap that can be reused in the manufacturing of steel.
The need for high efficiency removal of especially Zn from steel scrap has increased lately because the production of galvanized steel has increased enormously in the last 20 years, particularly in the building industry and in the automotive industry as well as for donestic appliances. In the life cycle sooner or later such Zn-containing steel will form scrap which is to be reprocessed in a steel making process for which only a limited Zn content of the scrap is allowed or desired.
In a known process for dissolving metals in an aqueous electrolyte for preparing a solution for plating purposes electrical power is fed to the electrolytical process in order to obtain an acceptable dissolution rate. In order to obviate electrodeposition of the metal being dissolved in the aqueous electrolyte on the cathode, a membrane may be installed separating the anolyte and the catholyte.
In dezincing or detinning metal scrap which takes place in an alkaline electrolyte for selectivity, dissolution is e.g. carried out without electrical power supplied making use of intrinsic galvanic coupling between different metals in the scrap in which case dissolution rates are very low, or electrical power is supplied from an external source such as a rectifier to the process.
U.S. Pat. No. 5,302,260 discloses a method of removing zinc from ganvalized steel without substantial co-dissolution of substrate iron comprising immersing the ganvalized steel in a caustic electrolyte solution anid electrically connecting the ganvalized steel to a cathode material which is stable in the caustic electrolyte and has a low hydrogen voltage.
EP 0 479 326 discloses alkaline leaching for dezincing of ganvalized steel scrap wherein the leaching solution includes an oxidant.